This proposal describes a neurophysiological investigation of the mechanisms of orofacial pain and analgesia for the purpose of generating information upon which a unifying hypothesis of sensory transmission within the trigeminal nuclei can be built. Current evidence suggests that significant modification of sensory messages in the trigeminal nuclei by both the central nervous system and intranuclear mechanisms can occur. By employing descriptions of the microstructure of the trigeminal nuclei as a guide for placement of electrodes, recordings of electrical activity in the trigeminal nuclei under varying physiological and drug induced states will be evaluated to determine the nature of the data transformation (i.e., the transfer function) at these loci. Because pain is a conscious sensation a chronically implanted rat preparation will be employed. A neurometeric anaysis utilizing EEG, evoked potential and multiple unit recordings will be undertaken. Treatments influencing pain transmission to be evaluated are: locally microinjected analgesics, focal brain stimulation and electrolytic lesions. Changes in bioelectric activity after tooth pulp stimulation will be correlated with changes in behavior after tooth pulp stimulation previously characterized. Upon completion of these studies a clearer understanding of the mechanisms of orofacial pain and analgesia can be developed which will aid the clinician in treating pain more rationally.